The Spitzer Space Telescope finally used up its liquid helium cryogen, but its …

As astronauts work to repair and upgrade the Hubble Space Telescope this week, another large space telescope of NASA started a new life on its own. As expected, on May 15th, 2009, the Spitzer Space Telescope ran out of its liquid helium coolant that has enabled it to observe dark, cold objects both within and outside of the solar system with ultra-high sensitivity. This doesn't mean that the telescope is nearing the end of its life; rather, it merely switches the mission from the “cold” phase it has been in since launch to a new "warm" phase, with new scientific objectives.

This is actually a big week for infrared astronomy. The European Space Agency launched two space-based IR telescopes of its own, the Herschel Space Observatory (HSO) and the Planck mission. The HSO is armed with the largest mirror ever put into space, at 3.5m, approximately four times the size of Spitzer's. It's carrying three years' worth of coolant that will allow it to image deeper into the infrared, pushing close to radio astronomy wavelengths. With the Spitzer now shifting to the shorter-wavelength ranges of the IR, the HSO and Spitzer missions will largely complement each other. The Planck satellite also observes in the far-infrared, but its goal is to measure the cosmic microwave background radiation. Both HSO and Planck are on a four-month trip to the L2 Lagrange point, during which time ESA scientists should have the chance to calibrate their instruments.

The Spitzer Space Telescope covers the infrared part of the spectrum. Astronomical objects that shine primarily in infrared are mostly cold, dark objects. To observe these stellar bodies, a telescope’s detector must be as cold as possible, as the thermal noise within the detector itself can otherwise overwhelm the very photons that come from the objects of interest. The Spitzer has been peering at those cold cosmic targets by chilling its light detectors to just 5.5 Kelvin above absolute zero using liquid helium, the coldest cryogen that we have.

The telescope was launched on August 25th, 2003, carrying 360 litters (96 gallons) of the cryogen, which was originally meant to last for at least 2.5 years with a hope of stretching it to 5 years. Achieving almost 6 years of cold-phase mission is beyond these original expectations. Even without the cryogen, the Spitzer’s capabilities in the mid-infrared are not expected to degrade at all, as that detector is designed to maintain temperatures below 30 Kelvin even without the liquid helium. These capabilities will not be surpassed by NASA until the James Webb Space Telescope becomes operational sometime after 2013.

What the telescope will lose is its ability to see objects in far-infrared—it can no longer see some of the darkest objects it revealed during the cold phase. Unlike the Hubble, which is in low-Earth orbit, the Spitzer orbits around the sun just behind Earth, and cannot be reached by the Shuttle to replenish the supplies and upgrade the instruments.

In the new warm-phase mission, the telescope will still focus on scientific objectives that remain out of reach of any other telescope. First, it will conduct surveys of the sky to map the universe at extreme distances. The light emitted by such faraway objects is redshifted to the infrared region. One of the expected results of the new Spitzer survey is the identification of more than a thousand quasars that are at least 12 billion light-years away.

Second, it will look for brown dwarfs in the cosmic neighborhood around our sun. Current theories predict that there should be about equal numbers of stars and brown dwarfs in our galaxy; however, very few of them have been observed to date, and the missing ones are believed to be too dark for most observatories to see. It is hoped that Spitzer will be able to find a dozen or so these brown dwarfs.

Astronomers will also try to use the telescope to see new stars form and grow—many of those young stars are hidden behind thick clouds of dust and extremely difficult to observe. Because the Spitzer observes in the infrared, which can penetrate dust clouds relatively unimpeded, it should reveal the dynamic changes that go on in those young stars as they start shining with light of their own.

And, of course, the Spitzer will continue to study planets inside and outside our solar system. The telescope has been extremely useful in monitoring the ongoing changes in the atmospheres of Uranus and Neptune. Its infrared observations can help determine the compositions of many small, icy bodies in the outer solar system and beyond Pluto’s orbit. Outside our solar system, astronomers will use the increased time allotted to mid-infrared observations to improve measurements of the temperature and wind distribution of the planets found so far around the other stars. It will also try to detect large, rocky planets, the so-called "Super Earths."

NASA has been planning for when the cryogen supply runs out. The warm mission depends on no consumables, and it is expected to last for at least 5 more years, running until the performance of its onboard instruments starts degrading. NASA has already scheduled over 10,000 hours of scientific observations for the first two years of the warm phase mission, and these plans will be executed as soon as the scientists and engineers make sure that the telescope behaves as expected without the cryogen.

Yeah, they utilize a cryogen to keep the sensor cold. This cuts down the thermal noise and allows a much more sensitive instrument.

The extreme temperature variation (300K on assembly to 5K operation) also creates interesting problems for the mounting of the sensor. I've seen pictures of a couple of CCD's that fractured into small pieces during cooling due to the thermal stresses combined with mounting forces.

Thanks Chuckles. Yeah, I can believe it, given that helium is used for plenty of physics experiments on earth... it's just that I'd never thought about anyone launching the super-cold stuff into space. I tend to think of space as being pretty cold to begin with (er, in the shade at least).

Now I'm curious: couldn't they carry some other, common compressed gas (nitrogen?) onboard the craft, then slowly vent it into space and use the decompression to cool the parts back down? Of course, eventually they would run out of that too, and if the problem is helium leaking out moreso than heating up, it wouldn't help. And maybe it wouldn't make economic sense to put the extra tank into space from the start...

Originally posted by mathrockbrock:Thanks Chuckles. Yeah, I can believe it, given that helium is used for plenty of physics experiments on earth... it's just that I'd never thought about anyone launching the super-cold stuff into space. I tend to think of space as being pretty cold to begin with (er, in the shade at least).

Now I'm curious: couldn't they carry some other, common compressed gas (nitrogen?) onboard the craft, then slowly vent it into space and use the decompression to cool the parts back down? Of course, eventually they would run out of that too, and if the problem is helium leaking out moreso than heating up, it wouldn't help. And maybe it wouldn't make economic sense to put the extra tank into space from the start...

The nitrogen is too warm. Liquid nitrogen boils at 70 K, the warm phase of Spitzer will operate at 25-30 K. So, yeah, space is pretty cold in the shade, just not cool enough to run the longer wavelength instruments.

In other IR astronomy news, NASA is scheduled to launch a survey satellite in Nov called WISE (Wide-field Infrared Survey Explorer). You can check out their web page here or the PI's, Ned Wright's, page here. It's going to operate at similar wavelengths as Spitzer (3.3, 4.7, 12, and 23 microns), but it will use a solid hydrogen cryostat for cooling and will image the entire sky several times over.

mathrockbrock:Liquid-He is lost not because it's passively leaking out, but because the detector is cooled by the latent heat of vaporization -- so, the liquid helium is actually boiled off to cool the detector. We could set up a multi-stage heat pump going as you're suggesting, (as some medical MRI systems do, I think?) that'd be very heavy and complicated. Less moving parts the more reliable the system becomes...

sunlight:On the sun-lit side, the Spitzer (And James Webb, too) have reflective coating to reflect sunlight. Between the coating and the telescope is an insulating space, and then the shadow side is all black to encourage cooling via blackbody radiation. When the incoming sunlight (of course, some heat is captured by the satellite) and the blackbody cooling is in equilibrium, this passive cooling is enough to keep temperature down to 30K -- it IS amazing.

And, here's a fantasy gedanken experiment: say, an object is in an intergalactic empty space and in radiative equilibrium. How cold will it be? Answer: 3 Kelvin -- it will be in radiative equilibrium with the 3K cosmic background radiation!